Devices for detecting radiation signals are already known and are used, for example, for determining the concentration of a gas using infrared absorption. Interfering gases in a gas volume, for example, in the interior of a motor vehicle, are, for example, CO2, CO, H2O or also CH4, carbon dioxide CO2 originating, in particular, from the respired air of persons in the gas volume or also from CO2 air-conditioning systems. Such interfering gases are detected using generally known gas detectors. Such gas detectors are also used in the area of laboratory analysis, safety technology, the food industry, air-conditioning systems, building technology, medicine, the household, environmental protection, etc. Different sensor principles, whose use depends essentially on the sensitivity, are used to determine the concentration of such interfering gases. For example, there are chemical sensors in which the resistance of a chemically reactive layer changes when it comes into contact with specific gases. However, these sensors are very sensitive, even to gases other than those that are to be detected, or also to environmental effects. Furthermore, the long-term stability of such sensors is a problem. Another physical method makes use of the different thermal conductivities of, for example, CO2 and air, that is, essentially nitrogen, in order to draw conclusions concerning the carbon dioxide content in a gas volume, such as in the interior of a motor vehicle. Furthermore, it is known that the absorption of infrared light by gas molecules may be used to detect such gas molecules. If, namely, a gas molecule, which is made up of several identical or different atoms, is stimulated by infrared light, it is converted to higher energy states. For example, rotational or vibrational moments are stimulated, whose energy state is specific to the molecule. By stimulating the higher energy states, energy is withdrawn from the optical radiation; the characteristic absorption bands of gases typically are in the infrared range for wavelengths between 1 μm and 10 μm. This means that infrared light passing through such a gas-filled space increasingly loses total intensity, the attenuation being a function of the gases present and of their concentration. The intensity of the light in the wavelength region of the absorption bands of the respective gases can be determined by a spectrally resolved measurement of the intensity. It is thereby possible to break down even mixtures of several gases with a high degree of resolution. An instrument for determining the gas concentration by infrared absorption according to the related art is shown in FIG. 1 and typically includes an infrared light source 20, a cuvette 30 or a tube 30, which is filled with the gas mixture to be analyzed and carries the light beam, and one or more filters 41, 42 which bring about the spectral dispersion of the infrared light, one or more detectors 51, 52 with which the intensity of the thermal radiation is measured after the spectral dispersion being provided behind filters 41, 42. However, such known devices have the disadvantage that they are expensive systems.